1. Field of the Invention
The present invention relates to a substrate developing method and a substrate developing unit.
2. Description of the Related Art
In a photolithography step of semiconductor device fabricating processes, resist coating treatment of applying a resist solution being, for example, a photosensitive resin to a surface of a wafer to form a resist film on the wafer, exposure processing of exposing a predetermined circuit pattern on the wafer on which this resist film is formed, and developing treatment of supplying a developing solution onto the wafer after the exposure processing to dissolve the resist film at exposed portions are performed in sequence.
In the abovementioned developing treatment, a strong alkaline developing solution is supplied onto the wafer, and is loaded on the wafer. Then, while this liquid loaded state is maintained for a predetermined period of time, the wafer is subjected to static development. In this static development, a part of the resist film, that is, exposed portions caused to have solubility in the developing solution by the exposure dissolve in the developing solution. When the static development for the predetermined period of time is finished, the wafer is rotated and a cleaning liquid, for example, a pure water is supplied onto the wafer so that the developing solution is replaced with the pure water. Thereafter, the supply of the pure water is stopped and the wafer is dried by liquid shaking-off by a centrifugal force so that a series of the developing treatment is finished.
In the actual static development described above, however, the resist film at all the exposed portions does not appropriately dissolve in the developing solution, and so-called insoluble substances which are in an insoluble state in the developing solution are precipitated in the developing solution from the exposed portions having been subjected to insufficient exposure, boundaries between the exposed portions given an insufficient exposure amount and unexposed portions, and so on.
Here, particles of insoluble substances dispersed in a liquid are usually electrically charged, and the electrically charged state of the surface of the particle (a surface potential) is generally evaluated by a zeta potential. The zeta potential represents a potential near the surface of the particle, and more precisely, is a potential at a part of a diffusion layer (sliding surface) on the periphery of a fixed layer formed around the particle in the liquid, with a position at infinity from the particle being a reference. Since the zeta potential of the particle is dependent on the polarity of the liquid surrounding the particle, it is influenced by the number of ions, that is, a pH value of the liquid. Generally, the zeta potential tends to be low in an alkaline liquid due to the existence of many negative ions while, in an acid liquid, it tends to be high due to the existence of many positive ions.
It has been confirmed from experiments conducted by the inventors that, in the above-described developing treatment, the zeta potential of the insoluble substances floating in the developing solution is negative due to the strong alkaline developing solution. It has been also confirmed that the zeta potential of the surface of the wafer in contact with the developing solution is also negatively charged.
The inventors has also confirmed that, when the developing solution is replaced with the pure water after the pure water is supplied onto the rotated wafer as described above, the pH value of the developing solution drastically lowers, resulting in the increase of the zeta potential of the insoluble substances toward 0 mV. Such decrease in the absolute value of the zeta potential of the insoluble substances weakens an electrostatic repulsive force between the insoluble substances and the surface of the wafer, and, in turn, relatively strengthens an intermolecular force to cause adhesion of the insoluble substances onto the surface of the wafer. Especially, when the absolute value of the zeta potential of the insoluble substances is decreased, the insoluble substances cohere together to grow into particles having a larger intermolecular force so that they easily adhere onto the wafer.
As a result, the insoluble substances adhering to the wafer cannot be removed easily even by the wafer cleaning at the time of rinsing, and the residual insoluble substances have been a cause of development defects. Further, it is necessary to keep supplying the pure water to the wafer for a long time in order to wash and remove the insoluble substances adhering to the surface of the wafer, resulting in increase in a total developing time, which lowers a throughput of the wafer treatment.
Further, a resist used in the above-described resist coating treatment includes a photoacid generator (PAG) which generates acid when being irradiated with light. For example, in a positive resist, a protecting group having an insolubilizing function in the developing solution and being releasable with acid is released therefrom due to the acid generated by the exposure, so that the exposed portions thereof are caused to have solubility in the developing solution. In a negative resist, the acid generated by the exposure induces a cross-linking reaction of a resin soluble in the developing solution, and thus the exposed portions thereof are caused to have insolubility in the developing solution. Then, by the supply of the developing solution in the above-described developing treatment, the exposed portions dissolve in the developing solution in the case of the positive resist while, in the case of the negative resist, the unexposed portions dissolve in the developing solution with the exposed portions kept undissolved, so that a predetermined resist pattern is formed on the wafer.
Actually, however, a sufficient amount of exposure is not given to the boundaries between the exposed portions and the unexposed portions in a chemical point of view. This means that the boundary is inferior in solubility in the developing solution. Further, in the case of, for example, the positive resist, the surface portions of even the unexposed portions of the resist slightly dissolve in the developing solution. So-called film-reduced portions of the unexposed portions are extremely inferior in solubility in the developing solution so that they are easily precipitated in the developing solution. Such existence of the portions inferior in solubility in the developing solution causes, for example, resist polymers half-released from the protecting group to float in the developing solution and, due to the cohesion or the like of the resist polymers, these resist polymers thereafter grow into resist particles, which sometimes adhere to the wafer. This re-adhesion of the resist particles causes development defects and prevents proper developing treatment. Moreover, in this case, a sufficient cleaning time is required for removing the adhering resist particles, resulting in an increase in a total developing time, which lowers throughput.
In a developing step in the above-described developing treatment, a predetermined portions of the resist film, for example, the exposed portions are dissolved due to the developing solution, and the insoluble substances which are kept undissolved and float in the developing solution are rinsed by the pure water in a cleaning step.
However, when the pure water is supplied onto the wafer after the aforesaid static development, the developing solution on the wafer is diluted so that the pH value of the liquid on the wafer drastically lowers to be close to neutrality. When the liquid on the wafer thus becomes closer to neutrality, the zeta potential of the insoluble substances, for example, the resist particles dispersed in this liquid becomes closer to 0 mV.
Since the decrease in the absolute value of the zeta potential of the insoluble substances lowers the electrical repulsive force among the insoluble substances, the insoluble substances cohere together to grow the particle size of the insoluble substances. The insoluble substances whose particle size has grown turn into impurities liable to influence the developing treatment, which has been a cause of the development defects.
Moreover, when the insoluble substances cohere together, they easily adhere to the wafer and the resist film. It can be reasoned that this adhesion is caused because an intermolecular force among the insoluble substances is strengthened. The insoluble substances once adhering onto the wafer and so on cannot be removed easily even when the wafer is rotated in an attempt to shake them off by the centrifugal force, and the residual insoluble substances have been a cause of the development defects. In addition, the pure water needs to be kept supplied to the wafer for a long time in order to remove the adhering insoluble substances from the surface of the wafer and so on.